1. Field
The presently disclosed subject matter relates to a manufacturing method of surface mount devices including LEDs, laser diodes, etc.
2. Description of the Related Art
FIG. 4 shows a conventional manufacturing process for multiple surface mount LED devices according to one cycle. The process includes: FIG. 4(a) preparing a circuit board 52, by forming conductor patterns 51 on a surface of an insulating board 50; FIG. 4(b) mounting LED chips 54 at predetermined intervals on the respective die bonding pads 53 of the conductor patterns 51 via a conductive material or the like; FIG. 4(c) connecting an upper electrode of each LED chip 54 to the corresponding wire bonding pad of the conductor patterns 51 via a bonding wire 55; FIGS. 4(d) and (d′) placing the circuit board 52 with the LED chips 54 mounted thereon in a mold 56, and encapsulating the LED chips 54 and the bonding wire 55 with an encapsulating resin 57 by way of resin transfer molding; FIG. 4(e) displacing the circuit board 52 from the mold 56, leaving the circuit board 52 in a normal/ambient temperature, and placing or forming a plurality of surface mount LED devices on the circuit board 52, in a state that the LED chips 54 and the bonding wires 55 are integrated by the encapsulating resin 57.
The circuit board 52 is diced by a dicing blade of a dicing machine at the predetermined intervals as shown in FIG. 5, with the encapsulating resin 57 being diced at the same time. The surface mount LED devices are respectively separated and completed as described in further detail, for example, in Japanese Patent Application Laid Open JP2000-196000 and its English translation, which are hereby incorporated in their entirety by reference.
In the manufacturing process of the surface mount LED devices described above, the circuit board 52 is sometimes warped, as shown in FIG. 4(e). The warp may be due to the formation process of the encapsulation resin. The warp can result in some problems in the last process for dicing the circuit board 52 and the encapsulating resin 57.
A more specific description of the warp will now be given. A thermosetting resin such as an epoxy resin is generally used for the encapsulating resin 57 which encapsulates the LED chips 54 and the bonding wires 55. The transfer mold for the encapsulating resin 57 is carried out by a heating process.
A characteristic of surface mount LED devices is that they are typically very thin devices. For the production of the very thin device, the circuit board 52 is also usually very thin, and includes LED chips 54 mounted on the mold bonding pads and bonding wires 55 connected to the wire bonding pads. For instance, a thin circuit board of 0.1 mm or less can be used for producing a thin surface mount LED device of 0.5 mm or less.
In the injection molding/transfer molding process for the thin circuit board 52 can include heating the insulating board 50, the conductor patterns 51, and the encapsulating resin 57 within the range of 100° C. to 200° C. and afterwards leaving the product in a normal or ambient temperature.
The coefficient of thermal expansion of the encapsulating resin heated within the range of 100° C. to 200° C. can be larger than the coefficient of the conductor patterns 51 provided on the surface of the insulating board 50. Thus, during the period when the device is left in the normal or ambient temperature, the encapsulating resin 57 shrinks more than the conductor patterns 51. Thus, the circuit board 52 can be curved such that it is concave in the direction of mounting the LED chips thereon, as shown in FIG. 4(e).
More specifically, in the case of molding a circuit board 52 that includes electrodes 51 (e.g. Cu/Ni/Au) formed on the insulating board 50 (for example, BT resin: made by Mitsubishi Gas Chemical Company, Inc) by the encapsulating resin 57 (for example, epoxy resin), the coefficient of thermal expansion of the insulating board 50 is almost equal to the coefficient of the encapsulating resin 57. The coefficient is about 6×10−5 at 100° C. and approximately 15×10−5 at 150° C. The coefficient of thermal expansion of the conductor patterns 51 (mainly Cu) formed on the surface of the insulating board 50 is approximately 1×10−5 or less, even at 150° C.
Thus, the actual value of the coefficient of thermal expansion of the circuit board 52 that is composed of the insulating board 50 and the conductor patterns 51 is closer to the coefficient of the conductor patterns 51 than the coefficient of the insulating board 50 under the influence of the conductor patterns 51. Thus, a difference of the coefficient of thermal expansion between the circuit board 52 and the encapsulating resin 57 may result. This can be one of the reasons why the circuit board 52 has a warp generated during the process of being left in the normal or ambient temperature after the injection molding process.
The thickness of the conductor patterns 51 (from 30 μm to 40 cm in general) is not easily changed. Thus, the mechanical strength of the insulating board 50 weakens in accordance with thinning of the insulating board 50. The coefficient of thermal expansion of the circuit board 52 itself is closer to the coefficient of the conductor patterns 51 because of the increasing influence of the coefficient of the conductor patterns 51. As a result, the difference of the coefficient of thermal expansion between the circuit board 52 and the encapsulating resin 57 increases, and the warp of the circuit board 52 accordingly becomes greater during the process portion at which the device is left in a normal or ambient temperature.
When the circuit board 52 with the warp as described above is diced in the dicing process, the circuit board 52 may not be diced at a right angle to the edge of the dicing blade of the dicing machine. As a result, the edge of the dicing blade can be easily damaged, and the contour of some or all of the surface mount LED devices that are diced and separated become imperfect in shape. Thus, when dicing a circuit board 52 that includes the above-described warp after the injection molding process, the circuit board 52 with the warp is typically corrected, maintained in a plane, and diced. When the correction is carried out in a normal or ambient temperature, because the thickness of the encapsulating resin 57 is very thin, problems such as cracking of the encapsulating resin 57 and peeling of the encapsulating resin 57 from the circuit board 52 may be caused.
As a measure for countering the above noted characteristics and problems, a conventional method has included: reducing the warp by using a flexible resin for the encapsulating resin 57; adhering a dicing sheet (PET sheet with adhesive material on one side) to the circuit board 52; holding the circuit board 52 by vacuum suction on a stage of a dicing machine; and dicing the circuit board 52 and the encapsulating resin 57 using the dicing blade of the dicing machine. Another conventional method includes: heating the circuit board 52 on which LED chips 54 and the bonding wires 55 are encapsulated by the resin; adhering the circuit board 52 to the dicing sheet when in a state that the encapsulating resin is softened; and dicing the circuit board 52 while in a state of being held by vacuum suction on a stage of the dicing machine.
The method described above of encapsulating with the flexible resin may result in products with low reliability because of the weak strength for the surface mount LED devices.
The method of adhering the circuit board 52 with the warp thereof in the state of heating it to the dicing sheet requires heating equipment and also requires additional man-hours. In case of insufficient heating, problems such as cracking of the encapsulating resin 57 and peeling of the encapsulating resin 57 from the circuit board 52 may be caused when adhering the circuit board 52 to the dicing sheet when taking the warp-corrective actions. The circuit board 52 is heated at the time of adhesion to the dicing sheet, however the circuit board 52 is also held in a state of correcting the warp by vacuum suction on the stage of the dicing machine in the normal temperature during dicing. Thus, each surface mount LED device that is diced and separated has the possibility of peeling off from the dicing sheet and scattering by impact at the time of dicing.
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, an embodiment of the disclosed subject matter can include a manufacturing method of a circuit board on which a plurality of surface mount LED devices are formed in which warp of the circuit board 52 can be reduced so as not to cause various problems in the dicing process of the circuit board 52, and to reduce or change other associated problems and characteristics of the conventional methods.